In the last 25 years since nanoparticles were first created, a single problem about them remained universal, namely their lack of cohesion. At sizes lower than 100 nanometers, these particles need to be organized in films, so that they can be used in mind-boggling applications such as flexible computer and TV displays, drug delivery, solid-state lighting, and semiconductor fabrication. Until now, these films would break up at the slightest of touches, but now experts at the Vanderbilt University have managed to find a way of making them strong enough to withstand handling.

In order for the films to remain tightly linked together, they have to be either encapsulated in a polymer coating, or mixed with chemical “cross-linkers,” molecules that ensure they stick together, acting very much like glue. However, “Adding this extra material can complicate the fabrication of nanoparticle films and make them more expensive. In addition, the added material, usually a polymer, can modify the physical properties that make these films so interesting,” VU Assistant Physics Professor James Dickerson, who has been the leader of the group that created nanoparticle films with no additives, says.

“Our films are so resilient that we can pick them up with a pair of tweezers and move them around on a surface without tearing. This makes it particularly easy to put them into microelectronic devices, such as computer chips,” the expert adds. Details of how the nanoparticle films stick together and what methods of construction the Vanderbilt team used can be found online in the May 28th issue of the renowned journal Chemical Communications.

In order to make their nanoparticle films so resistant, the researchers turned to a technique called electrophoretic deposition (EPD), in which particles move onto an electrode under the influence of electrical current. The process is widely used in coating things such as car parts, appliances and other similar objects with a layer of particles that prevent things like rust or the accumulation of static energy.

In order to use this process in nanoparticles (which the VU team is the first one to attempt), the scientists used a spun-cast layer of polymer, in a slightly modified version of EPD, called sacrificial layer electrophoretic deposition. The name comes from the fact that the polymer layer is used as a pattern for the nanoparticles to be drawn to each other, after which they are dissolved (sacrificed), and the nanoparticle layers emerge tightly bound together.

One reason why these particles are so strong afterwards is the fact that the electrical field that goes through the liquid medium in which the process takes place packs the particles so closely and strongly together, that it allows for naturally occurring inter-particle attraction forces to come into play. “The technique is liberating because you can make these films from the materials you want and use them where you want,” Dickerson says.